Technical Field of the Invention
The present invention is directed to elastic strain and pressure sensors and associated devices and systems for measuring motion and contact. Specifically, the present invention is directed to a hyper-elastic strain sensor that can be used to create an artificial skin that measure motion and touch.
Description of the Prior Art
Emerging technologies such as wearable computing [1] and soft active orthotics [2] will depend on stretchable sensors that register deformation and surface pressure. These softer-than-skin sensors must remain functional when stretched to several times their rest length, avoid hysteresis and permanent deformation, and preserve the natural mechanics of the wearer or host system. Hyper-elastic transducers for strain and pressure sensing represent just one aspect of the much broader and potentially revolutionary fields of elastically stretchable electronics and computing.
Current approaches to stretchable electronics include buckled (wavy) films of semiconductors for stretchable circuits and diodes [3-5] as well as elastomers that are embedded with microchannels of conductive liquid [6-8]. The latter approach utilizes many of the same molding, embossing and lithography techniques that are used to fabricate soft microfluidic devices [9-11]. One advantage of elastomers is their hyper-elasticity, which allows for mechanical durability and stretches as great as 1000%. Such properties are particularly favorable in wearable devices such as adaptive orthotics and insoles that must sustain large deformations and pressures.
Previous efforts in soft pressure and strain sensing and so called artificial skin include capacitive sensors composed of an elastic insulator layered between conductive fabric [12-14] or a silicone rubber sheet embedded with thin gold film [15]. Other efforts include resistive sensors composed of elastomer embedded with conductive microparticle filler [16-18] or ionic liquid [19-21] and a flexible artificial skin embedded with semiconductor nanowires [22].
Prior designs for pressure sensing are adapted from the Whitney strain gauge, which was introduced in 1949 to measure the change in circumferential girth of muscles and limbs [23, 24]. The original Whitney strain gauge was composed of a rubber tube filled with mercury and used a Wheatstone bridge to measure the change in electric resistance of the mercury channel corresponding to stretch. Recently, this principle has been extended to stretchable microelectronics, composed of eGaIn-filled microchannels embedded in polydimethylsiloxane (PDMS) rubber [6]. Embedded channels of eGaIn can also operate as a stretchable, mechanically tunable antenna [7] or as strain sensors [8] for measuring stretches of as much as 200%.